Fix resource leaks in Input Actions Editor window.

[AlexTyrer]

Description

This PR addresses resource leaks in the Input Actions Editor window.

These resource leaks could lead to unexpected exceptions being thrown.

Changes made

InputActionsEditorSettingsProvider is now handled by a single static instance to work around UIToolkit callback order issues identified.

Fix VisualElement handling on the StateContainer that manages the tracked serialized object representing the Input Actions.

Checklist

Before review:

• [x] Changelog entry added.

  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.

• [x] Tests added/changed, if applicable.

  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.

• [x] Docs for new/changed API's.

  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

• [x] Commit message for squash-merge is prefixed with one of the list:

  • FIX: Fixed Input Actions Editor window resource leak that could result in unexpected exceptions ISXB-865

[ekcoh]

I think this can land after QA assessment is available with approval.